import re
def isNumLeters(s):
    if s == '':
        return False
    if 6<=len(s)<=18 and re.match('^[a-zA-Z_]+$',s[0]) and re.match('^[0-9a-zA-Z_]+$',s[1:]):
        return s
    else:
        return False



username = ['asd123456_','4564wa','waesr489798','_wdqwdas48494']
ret = list(map(isNumLeters,username))
print(ret)

ret = list(filter(isNumLeters,username))
print(ret)

from functools import reduce
lst1 = [1,2,3,4,5,6]
ret = reduce(lambda x,y:x*y,lst1)
print(ret)

ret = list(filter(lambda x:x%3 == 0,range(1,100)))
print(ret)

ret = list(map(lambda x:x*x,range(1,10)))
print(ret)

lst1 = [1,4,5,9]
lst2 = [str(x) for x in lst1]
result = ''.join(lst2)
print(result)